Method and apparatus for dielectric heating



Ot. 24, 1950 w, sc JR 2,526,598

METHOD AND APPARATUS FOR DIELECTRIC HEATING Filed Jun. 21, 1946 v 2 Sheets-Sheet 1 INVENTOR: Geory e W 0 T, J1.

Oct. 24, 1950 -G. W. SCOTT, JR

METHOD AND APPARATUS FOR DIELECTRIC HEATING Filod' Jun 21,

2 Sheets-She'et 2 fig. 2. x

I INVENTOR. George M Sea/'1: Jr:

97 4M TORNEYS.

Patented Oct. 24, 1950 METHOD AND APPARATUS FOR DIELECTRIC HEATING George W. .Scott, ,lr., Lancaster, Pa., assignorto Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Application-June 21, 1946, Serial No. 678;,2'15

15 Claims.

This invention relates 1 b} the. heating... of electricaltinsulating .material by .the dielectric effect.

and, inv particular. to-a container. ormold for confining insulating material in thelform of.1oose particlesduring such heating in which the charge of materialand themold are bothxheated simultaneously by -the dielectric eifect.

The heating. of. various materials .by the dielectric effect has. comeeinto. extensive use :in. recent years. Whenthe material to be heated is in the formeof largesolid masses, ve. g., laminated wood board, little.v orno difficulty is encountered in maintainingitin proper: relation to the spaced. electrodes. by which the high-frequency; electric fieldiis applied A different. situation prevails, however, in the caseof :materialin theform of loosemarticlesofsmall size; which flowarather freely, such as cork granules lightly coated with a. heat-activatable binder. ,In .orderto process such;..material;.into a solid. block; by dielectric heating, they 'mustqnecessarlly be confinedsince pressure must be; applied, to producea cured mass of.the desired density.-. Nolcommercially'satisfactory mold has. beengavailable heretofore for confining loose particles underv pressure during proce sin v eleet i etlne and t is heyobe iectrofmy inventiento p ev esuch a mold.

Ihe characteristics; desired in, a mold for; use;

in processingmaterialby dielectric heating are w 11..-k .0wn-. but, se rla liem aware. -k wn construction of :mddposses e sufi c en o these characteristics; to make it satisfactory for commercial production. ,In; the first .,place,- the mold walls :must have high electrical insulating value to withstandgthe voltage applied across the electrodes betw e whic t emete a to be heated.

is disposed: The moldwallsmust alsohave con-g siderable mechanical strength, not only to with stand the; pressure; applied tothe material dur ing processingbut alsotherelatively rough usage oiawhi h any appara sin lv deinfi tory; pro: duction is always subjected. 4 In addition, the material used in the;construction of the mold should be easily machinable to facilitate fabrication.

Wood naturally suggests itself as a material for mold walls capable of meetingtheserequirements but experiencehaS shown that it alsohas shortcomingswhich make, it quite unsuitable for thepurpose. Even the densest woods are porous to a degree andoontain more or lessmoisture. Although wood ca-nbe dried out quite thoroughly by appropriate treatment, it re-absorbs moisture from the atmosphere, because of its cellular structure, orfromthe material being processed if it containsmoisture. The amount of moisture pre e t.in.,w d ...furt e more, varie from day to day with atmospheric conditions. This causes variations in the loss;factor which is ameasure .of the likelihood ofpheat being generated internally when-the wood is subjected to an electric field; of; a given voltage alternating at a given frequency. Tobe eee table commercially, the materialforming the walls of amold for dielectric p o es nemust hev .el.l ss f c within e- I tivelyfnarrow limits, If the factor is too high,

arrexcessiveamount of heat will be generated in the walls resulting in a Waste of electrical energy and possibly in injury to the walls themselves.

,Ifythe factor istoo low, the heat generated in the mold wall is not -sufiicient to bring its tem perature; up to that which is desired in the mass 7 of material under treatment with the result that the mold wall cools the lateral surface of the. mass. A further objection to wood for mold walls I is that such moisture as is absorbed thereby is vaporized under the heat generated in the processing of the material to be treated, causing the wood towarp, split, crack or check and impairing or destroying its usefulness. In addition, Wood mold Walls, because of theabsorbed moisture present therein, are susceptible to arcing which leaves carbonized paths throughthem and reduces their insulation value, For the foregoing reasons, wood as. such has been abandoned, de-

spite its apparent desirability, as a material for the walls of a mold for dielectric .processing. Other materials such as ceramics andvolcanlc ash havebeen tried but without such success'as to indicate any prospect of commercial adoption.

I have discovered tha't -a highly satisfactory mold for dielectric processing mfiy be made from hard-wood which hasbeen impregnated with a wax having special properties. Specifically, I employ ceresin or its equivalent as the impregnant for moldwalls of hard wood such as maple,

.elm, birch,beech, oak or hickory. Other waxes having similar qualities, or amixture thereof with ceresin, may ,also be employed. A mold constructed according to my invention has high insulation value at all'temperatures encountered in,

curing heat-activatable; mines, a proper. loss factor, is moisture-free and uniformly resistant to absorption,,regardless of varying atmospheric conditions, and isnot. subject to warping or dis-- tortion on repeated. heating and cooling. The impregnantis non-volatile, non-arcing and hasa melting. point high enough so that it does not cording to my invention comprises a box open at the top and bottom and having side walls ll of impregnated hard wood. As shown, the side Walls II are preferably disposed with the grain thereof vertical. While the mold has been shown as of rectangular or square shape. in plan, it may be of any other desired shape. The side walls II are secured in assembled relation by any suitable means, conveniently angle iron frames 12 and I3 at the top and bottom of the mold. These frames are composed of suitable lengths of angle iron welded together. The side walls are secured to the frames by wood screws [4 extending through the latter and into the former.

The mold I0 is adapted to confine a charge of material [5 during processing by dielectric heating.- The charge may, for example, be cork granules coated with aglue-glycerine binder, a

'resinbinder, latex, or the like adapted to be activated or cured by heat and converted into a solid mass; A plate [6 serves as a bottom for the mold. It rests on transverse pins l1 removably inserted through holes in opposite sides of the mold and the-frame l3. For-filling the mold with the charge, I employ a boot removably disposed on topof the'mold, since the volume of the material when charged is considerably greater than when compressed within the mold for heating by the dielectric effect. The charge is compressed by applying pressure to a top plate I8 similar to the plate 16, disposed on top of the charge, and forcing it' downwardly through the boot and into the mold to its finalposition in which it is held by pins [9 removably inserted through holes in the opposite side walls and the frame [2, in the same manner as the pins 11. The plates l6 and 18 serve not only as the bottom and top of the mold for confining the charge under pressure but also as condenser electrodes, having connections 28 to a suitable source of high-frequency alternating current.

Ihecurrent source is a conventional electronic oscillator delivering a frequency of about megacycles and a voltage of about 1.0 kilovolt per inch of thickness of the charge [5 when compressed to final volume.

The mold is provided with upper and lower peripheral conductors 2| connected to the plates I'Sand l6, respectively, by straps 22 and 23 se-' cured to the frames 12 and I3. The conductors 2| cause the portions of the charge adjacent the lateral surfaces to be heated to a temperature higher than that attained by'the interior of the mass to compensate for the cooling effect to which'the lateral portions are subjected.

The mold as described so far is also disclosed and claimed in my copending application, :Serial No. 678,214 filed June 21, 1946 for Dielectric Heating Method and Apparatus.

I The side walls ll of the mold are composed of hard wood, such as maple, birch, beech, hickory, oak or elm, which should be straight-grained and free of knots or other irregularities. As previously stated, the pieces composing the side walls are preferably disposed with their grain extending vertically. The boards from which the pieces are out should not be heart wood. In making the mold, the pieces are thoroughly dried to reduce the moisture content thereof as low as possible. The pieces are then subjected to vacuum impregnation in a bath of awe]; of suitable characteristics, heated to a temperature materially above its melting point and preferably in the neighborhood of, but below, its boiling point. The wood being practically bone dry is thoroughly impregnated by thewax and any residual moisture is vaporized and displaced. The wood pieces are left immersed for a sufficient time to insure thorough impregnation. After such time has elapsed, the pieces are removed, drained and, after, cooling and solidification of the impregnated wax, they may be assembled into a mold as shown in the drawings.

The wax employed for impregnating the wood mold walls may be any one of a number of compounds. I have found, however, that ceresin has special properties which make it particularly de-..

sirable for this purpose. Ceresin or purified ozokerite, as :it issometimes called, is a white mineral wax which maybe obtained with a melting point as high as 175 F. It isvsoluble in common hydrocarbons'but is insoluble in water. It thus renders the. impregnated. side walls 1 thoroughly water repellent; preventing any absorption of moisture either from the atmosphere or from the charge should it contain any moisture. ceresinaimpregnated woodwhen subjected to a high-frequency. electric field, exhibits a relatively low loss factor, i. e., K tan. a, where K is the dielectric constant'and tan 5 the dissipation factor. This-expression may also .be written as K cos 0, where cos 0iis the power factor, if .the latter is .1 or less. The actual values for ceresin are K, 2.14 and tan a, .002 or less, at a frequency of 10 megacycles and room temperature, giving a loss factor of .004 or less. For maple wood impregnated with ceresin, the value for K is 2.8 and for tan 6, .042, at 10 megacycles and room temperature, giving a lossfactor of .116. This value proves particularly desirable for the walls of a mold to be used in a particular practice of my invention, i. e., processing cork granules bonded with a mixture of glue and glycerin activated by paraformaldehyde parts of cork,

6 parts of glue,'20 parts ofglycerin, 2 parts of water and '-part of formaldehyde, by weight). Such a loss factor is desirable because it avoids overheating the mold walls and the waste of energy incident thereto but ishigh enough to cause the mold walls to be heated to the proper temperature. Overheating of the mold walls might result-in overcuring of the portions of the charge adjacent the lateral surfaces and perhaps in charring of the wood itself-.' The loss factor of ceresin-impregnated maple, .116, as-

(.166) than a mass of phenol-formaldehyde molding powder having a filler of wood flour therein. This material has a loss factor of .156 at 10 megacycles and room temperature. Similarly, a urea.

1946, and entitled Method of Compensation fen or-=-Preventing Heat Losses from MaterialDuring Dielectric Heating Thereof The following tables furnish data on the dielectricpropertiesof-a group of hardwoods, im-

pregn'ants, and hard woods impregnated with.

various impregnants.

Table I Dielectric Values Wood 1 Loss Factor Oak (dry) .057 Hickory (dry) .065 Birch (dry) 1 Measurements made with grain perpendicular to electrodes.

Table II Dielectric values Melting Ixnpregnant Po h? L K Tan 5 Factor ceresin,.. 130-175 2.14 p.002 i Paraffin 122 2. 08 5.003 g 006 Beeswax and Rosin" 145-158 '2. 3'1 .008. 010 010-023 Superla 165 2 23 001-. 003 002*, 007 Halowax 1012 203-210 3. 38 .034 115 Halowax 1013. 2 i7-252 4. 36 061 266 Halowax-IOM. 277283 3.84 .048 .184

Table III Mold material 7 Dielectric values Wood 1 Impregnant K Tan Maple (dry) Ocrcsm 2. 84 .0 12 .116

Maple (79% R. 11).. do.... 4. 2 .065 .27 Maple (dry) Parafiin 2. 84 .036 .101 Do Superla 2. 9 .043 .125

Maple(97% R. 5.0 .057 .20 Maple (dry). Halowax 1012. 3. 2 .051 166 Maple 97% 3.11).. llll .do 5. 5 .078 .46

1 Measurements made with grain perpendicular to electrodes.

All values for K, Tangent 5, and Loss Factor were determined at a frequency of 10 megacyeles with the material maintained at 70 F. Throughout this specification and in the claims the loss factor expressed is so determined. The Halowaxes are chlorinated naphthalene compounds and Superla is petroleum byproduct hydrocarbon wax.

As pointed out above, the optimum loss factor of a wax impregnated wood mold for cork composition manufacture is in the order of .116, and for best results, it is preferred to have the loss factor of. thetreated wood fall within the range of .1 to .25. It is possible, however, to utilize impregnated wood molds having'a c-sr; factor great as .4. For speclalmoldingfoperations, aioss;

factor somewhat above .4 may be desirable dee pending upon the geometry of the mold, thenature of the productbeing molded and heated, and

other variable factors. Generally, however, a loss.

factor greater'than .25 will not be necessary. By suitable combinations'of wood and impregnant, or mixtures of impregnants, the desired loss factor may be obtained. For example, from Table I it may be determined that dry hickory-has a loss factor of .18 and from Table II that Halowax' 101 31has a loss factor of .266. By impregnating. hickory" with Halcwax 1013', a moldwallhaving a loss factor between .20 and .25 may be obtained.

It must be remembered that the use of. an impregnant will increase the loss factor for any wood in dry condition, for the wood is constituted with considerable void space normally filled with air which is replaced by the impregnant which has a higher'loss factor than air.

Tables I and III indicate clearly the effect of moisture on the loss factor of wooden mold materials. Maple wood, for example, maintained at 97% relative humidity for thirty days had a loss factor of 1.49 as contrasted with a loss factor of but .16 for dry maple and .116 for ceresin impregnated maple. The loss factor of maple. impregnated with ceresin and held at 79% relative humidity for thirty days is .27 andwhen impregnated with Superla and held at 97% relative humidity for the same time is .29, as contrasted with untreated maple held at the same relative humidities for equal times which has loss factors of .55 and 1.49, respectively.

Figure 3 shows how the mold is utilized in heat-- is quickly heated by the dielectric effect as a result of the alternating electric field established between the plates. The heating effect is substantially uniform throughout the mass of material and it may thus be quickly brought to the temperature required for satisfactory curing of the heat-activatable binder, such as that men tioned above, resin, latex, or the like. i 7

Experience has shown. that a mold according to my invention provides a highly satisfactory solution for the problem of confining loose mate- -rial during dielectric heating which is characterized by numerous advantages. In the first place, the side walls of the mold composed of. hard wood thoroughly dried and impregnated with wax having the properties of ceresin, exhibit a high electrical insulation value and a low loss factor. As a result, the danger of overheating of the walls is minimized. The mold walls, furthermore, are characterized by electrical stability. The impregnant makes them uniformly moisture repellent so there is no substantial variation in the loss factor with normal atmospheric conditions. The high melting point of the wax and its lack of volatility prevent loss of impregnant from the wood during operation. The impregnant also renders the wood non-arcing so that the insulating property thereof is maintained indefinitely.

A mold with walls made in accordance with my invention is also characterized by mechanical stability. That is to say, the walls are not subject to warping, splitting or cracking as a result of the presence of moisture. The impregnating treatment vaporizesany residual moisture left in the wood after drying and prevents the re-absorption of moisture from the atmosphere or the material being processed. The invention also provides mold Walls which may be made at low cost and easily machined to provide the desired smooth surface with ample'mechanical strength and toughness for the service to which the mold is subjected in use.

Although I have illustrated and described but a preferred embodiment of the invention, it will be recognized that changes in the details thereof may be made without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. In a method of forming molded articles the steps including depositing a charge of dielectric material in a mold composed of dielectric material having a loss factor between .1 and .4, and subjecting the material and mold to a high-frequency electric field for simultaneous heating by the dielectric effect.

2. In a method of forming molded articles the steps including depositing a charge of cork granules bonded with a mixture of glue, glycerin and para-formaldehyde in a mold of hard wood impregnated with ceresin and subjecting the granules and mold to a high-frequency electric field for simultaneous heating by the dielectric effect.

3. In a method of forming molded articles, the steps including: depositing a charge of cork granules coated with a heat-activatable binder into a mold composed of dielectric material having a loss factor between .1 and .4 and subjecting the material and mold to a high-frequency field for simultaneous heating by the dielectric effect.

4. In a method of forming molded articles, the steps including: compressing a charge of cork granules coated with a heat-activatable binder in a mold of hardwood impregnated with wax, said mold having a loss factor between .1 and .4, subjecting the material and mold to a high-frequency field for simultaneous heating by the dielectric effect, and continuing the application of said field until said binder has been activated.

5. In a method of forming molded articles, the steps including: depositing a charge of dielectric material to be molded into a mold composed of substantially moisture-free wood impregnated with a nonvolatile Waterproofing impregnant substantially completely filling all of the void spaces within the wood .and imparting to the mold a dielectric loss factor between .1 and .4 and subjecting the charge and the mold to a highfrequency field for simultaneous heating by the dielectric efiect.

6. In a method of forming molded articles, the steps including: depositing a charge of cork granules coated with a heat-activatable binder into a mold of hard wood substantially free of moisture and substantially completely impregnated with a nonvolatile waterproofing impregnant, said mold having a loss factor between .1 and .25 and said charge having a substantially higher loss factor, and subjecting the charge and the mold to a high-frequency field for simultaneous heating by the dielectric effect to activate said binder.

7. In combination with a source of high-frequency current, a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneou ly, s id T3013. comprising side walls of hard wood impregnated with a waterproofing impregnant and having a loss factor between .1 and .4, and spaced electrodes connected to said source of high-frequency current and disposed to establish a high-frequency field in the mold and the charge for simultaneous heating of the mold and charge by the dielectric efiect.

8. In combination with a source of high-frequency current, a mold for confining a. charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, said-mold comprising side walls of hard wood substantially free of moisture and impregnated with ceresin, said mold walls having a loss factor between .1 and .4, and spaced electrodes connected to said source of high-frequency current and disposed to establish a high-frequency field in said mold and charge for simultaneous heating of the mold and the charge by the dielectric effect.

9. In combination with a source of high-frequency current, a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, said mold comprising side walls of hard wood substantially free of moisture and substantially completely impregnated with a wax, said mold walls having a loss factor between .1 and .4, and spaced electrodes connected to said source of high-frequency current and disposed to establish a high-frequency field in said mold and charge for simultaneous heating of the mold and charge by the dielectric effect.

10. In an apparatus for dielectric heating including a source of high-frequency current and a pair of spaced electrodes connected to said source: a mold for confining a charge of material disposed between said electrodes for processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, said mold comprising walls of hard wood substantially free of moisture and impregnated with a waterproofing material having a melting point above F. which substantially completely fills all of the void space within the wood, said mold walls having a dielectric loss factor between .1 and .4.

11. In combination with a source of high-frequency current, a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, said mold comprising side walls of hard wood substantially free of moisture and substantially completely impregnated with a waterproofing impregnant of wax having a melting point above F. and a loss factor between about .002 and .266 and imparting to the mold a dielectric loss factor between .1 and .4, and spaced electrodes connected to said source of high-frequency current and disposed to establish a highfrequency field in said mold and said charge for simultaneous heating of the mold and charge by the dielectric effect.

12. In combination with a source of high-frequency current, a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, said mold comprising side walls of hard wood substantially free of moisture and substantially completely impregnated with a waterproofing impregnant, said walls having a loss factor between .1 and .25, and spaced electrodes connected to said source of high-frequency current and disposed to establish a high-frequency field in both said mold walls and charge for simultaneous heating of the mold and charge by the dielectric efiect.

13. In combination with a source of high-frequency current, a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, said mold comprising side walls of maple substantially free of moisture and impregnated with a waterproofing impregnant consisting of ceresin having a melting point between 130 and 175 F. and a loss factor of.004 or less, said mold having a loss factor of about .116, and spaced electrodes connected to said source of high-frequency current disposed to establish a high-frequency field in said mold and charge for simultaneous heating of the mold and charge by the dielectric effect.

14. The combination of claim 7 in which the waterproofing impregnant is chlorinated naphthalene.

10 15. The combination of claim 7 in which the waterproofing impregnant is a hydrocarbon wax.

GEORGE W. SCOTT, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 29,625 Seiberling Aug. 14, 1860 1,377,241 Winestock May 10, 1921 1,464,044 Marr, Jr Aug. 7, 1923 1,560,172 Chamberlain Nov. 3, 1925 1,648,294 Coolidge, 3rd Nov. 8, 1927 2,296,948 Pitman Sept. 29, 1942 2,304,958 Rouy Dec. 15, 1942 2,316,752 Atkinson et al Apr. 20, 1943 2,322,903 Wilkoff June 29, 1943 2,325,637 Stewart Aug. 3, 1943 2,341,617 Hull Feb. 15, 1944 2,421,097 Lakso May 27, 1947 2,421,098 Vogt et a1 May 27, 1947 

